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Old 09-18-2017, 08:14 PM   #1
Join Date: Sep 2017
Posts: 63
DEF Capacity in Forester MBS 2401W

We are new to RVing and purchased our Forester in August. Dealer stressed the importance of maintaining DEF and it would last approx 5,000 miles.

At 2400 miles, our DEF light came on stating the fluid was low. We purchased 2.5 gallons of DEF at a truck stop and emptied the entire container accordingly.

I could not find in the owner's manual information about: 1) the capacity of the DEF tank, and 2) how to gauge how much fluid you currently have in the tank. Can someone please provide a response to both questions. Thank you!

(Needless to say, the dealership probably did not fill the DEF tank before we drove it off the lot.)
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Old 09-18-2017, 08:34 PM   #2
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I think it is 4 or 4.5 gallons. There is a way to scroll through the info display data to show you the level, use the buttons on the steering wheel.
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Old 09-18-2017, 11:21 PM   #3
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There are some variances when it comes to DEF reservoirs/gauges on the MBS. From my research most of the chassis upfitters (i.e. Forest River) are using is the Sprinter 519 Cab/Chassis which has a DEF Reservoir of 18 liters (4.755 gal). Older model years may be different and of course everything is subject to change in the future too.

I do believe that MBS model year 2016 was the first year in which there is a DEF gauge within the instrument panel. Accessing requires pushing some buttons. I've also since noticed that Forest River is now building the MBS models with the steering wheel controls, although I'm not sure when this change occurred as these changes usually occur on the fly as they order new inventory rather than from model year to model year.

There's been a lot of discussion, especially within the MBS and (now older) Solera sections of the forum regarding DEF, capacities and gauges.

Myself I have a 2016 Forester built on a 2015 MBS chassis. I've confirmed via VIN build lookups, manuals and from the sprinter-source forum that my reservoir is 18 liter, but I don't have any gauge to navigate to within the instrument panel and no steering wheel controls either. A little frustrating, but the DEF does seem to last a while. My first DEF light (to add DEF) was at about 6800mi. But I'm not sure if the dealer had topped it off or not. I do know that when I take it in to Freightliner for an annual service they top it off.

Anyway, hope this helps...
2016 Forester MBS 2401S
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Old 09-20-2017, 08:33 AM   #4
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On our 2016 you push the 'M' button to the left of the speedometer. Each time you push the button a new display will appear. Look for the display that has DEF at the top.
I fill mine at just past half empty. Usually takes 1 container to fill. This is about every 5000 miles. MB tech said always fill to full to reset the sensors and it is not good to let it get past 1/3 full.
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Old 09-20-2017, 09:01 AM   #5
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Extremely helpful! Thank you SO MUCH!
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Old 09-20-2017, 09:13 AM   #6
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Lifted from elsewhere,,

Here's my point of view for what it's worth. If you follow MB's recommendations and rely on the low DEF indicator you can have problems. Why? Because DEF is corrosive if it dries out on a sensor that now has air reaching it a corrosive environment can be created. The DEF can crust up on the sensor and start to corrode it. The safest thing to do is to always have the sensor submerged so it can't corrode. I top off my DEF every three or four tanks of fuel and it keeps the sensor under the DEF.
I look at it as preventive maintenance.
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Old 09-20-2017, 09:17 AM   #7
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Excellent feedback! Thank you so much!!!! I will do as you say and keep the sensor submerged.
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Old 09-20-2017, 12:14 PM   #8
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I top off my DEF fluid with every full tank of fuel. Low fluid levels can cause ammonia to crystallize on the walls and interior tank components. Don't want to risk any problems that that may cause.

DEF isn't a big deal. I get the cheap stuff from Walmart and carry a spare just with me wherever I go just in case.
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Old 09-20-2017, 01:15 PM   #9
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DEF -- we have the same FR2016 MBS Model

We've been cross country West Coast (San Diego, CA) to the East Coast (Outer Banks, then up to Manassas, VA) and back twice (2015 and 2016). 3 months on the road each time. Plus up and down the Pacific Coast here in CA. We've not had the DEF light come on. Maybe we are a bit over reactive (after decades of tenting, and Apache pop up, and two VW Campers), this is our first RV. My wife is German and we just don't waste money. We love the size of this RV. We just love this RV. We've put nearly 40K miles on it in 2 1/2 years. No, we aren't full timers. Retired military and we just love to travel. This RV fits our needs like a glove. Plus our local RV dealer is fantastic (family owned) and we have an extended warranty on the House and an extended warranty on MB itself and that MB service center is fantastic as well -- also here locally. We knew we'd be putting some miles on it. Ain't no point in spending that kind of money and letting it sit. In any event, we usually check the DEF at each RV overnight stop so long as the trip is around 300 miles one way. Perhaps every two days. It takes only about 5 minutes to open and eyeball it. We check the oil too. I carry a couple of oil top off quarts with us and 1-2 gals of DEF (they weigh a lot) because you actually can't get it everywhere if you take the scenic side tours.

My guess is your RV dealer didn't check your your DEF before you took possession. I'd check a few times before, during, and back off the road, before worrying about it. Just keep a DEF gallon at least on board so you don't get stuck somewhere. Oil doesn't weigh much in quart sizes -- keep a couple on board as well. Here's wishing you fantastic fun and safe travels.


Dan N Gerda (and Sprinkles)
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Old 09-20-2017, 01:44 PM   #10
Join Date: May 2017
Posts: 51
I have 2018 Isata and if you press left side of button on the steering wheel 3 times then up button 3 times it shows the DEF tank gauge. Warning light comes on below 1/2 full. I have read it is difficult to reset if you run all the way to low and engine drops to a 35 mph level. I get about 3,500 mile on 2.5 gallons. My tank holds 4.3 gallons.

I have started carrying the 1 gallon DEF container sold at Walmart since it takes less space then the 2.5 gallon version. I top off every 1,000. Of note - I have read DEF fluid is extremely corrosive and you don't want to spill it on your engine. Not easy to fill without overflowing so you have to use care and pour it in slowly.
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Old 09-20-2017, 04:35 PM   #11
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Location: Carlyss, Louisiana
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How much DEF a vehicle consumes depends on how and where you drive it. My 2015 Chevrolet 2500HD running around town and not towing anything will consume 1 gal. per 1000 miles. However, if I am towing our 5th wheel RV, then it consumes much more each 1000 miles. The best place to fill you DEF tank is a highly used truck stop, their DEF is always fresh because of the huge consumption by trucking companies. Watch out for the manufactures date on any that you purchase in the stores, shelf life under ideal conditions is only from one to two years. I do not fill my tank full unless I am going on a long trip, do not want it to go bad in the tank while truck is not being used.
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Old 09-20-2017, 05:36 PM   #12
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Thank you so much!
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Old 09-22-2017, 09:00 AM   #13
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I filled the def tank three times having measured the mileage between each fill-up.
I am comfortable to estimate that I use approx. 1 liter per thousand miles. Like I say, it's an estimate.
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Old 09-24-2017, 12:03 PM   #14
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DEF Capacity MBS 2400/2401

From the 2016 MBS Sprinter Owner's Manual

DEF supply reservoir content
Tank contents
Diesel Exhaust Fluid (DEF)11
Approximately 4.7 US gal (18.0 l)
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Old 09-26-2017, 08:58 AM   #15
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When we bought our 2017 FR Forseter in March the light came on while driving home from the dealer! We panicked, called back to the dealer and were told to add some. So we stopped and bought blue-def only to find out it was full! The problem was the mechanic when showing us the RV had unscrewed the top and not screwed it back on tightly! We have just returned home from a 7,000 mile cross country trip. We added it several times along the way (most gas stations selling diesel carry it, even on the back roads) but we too started caring an extra gallon. The gauge is on the info panel accessed by the scroll button.
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